This material is known as "watermelon calcite" or "watermelon rock" due to its colorful red and green fluorescent response under UV. This flat specimen consists of pale yellow calcite sandwiched between two layers of light grey chalcedony. Under short wave UV the chalcedony fluoresces green and the calcite layer red/orange, giving the appearance of a succulent piece of watermelon. This specimen was collected from the Buffalo Gap Grasslands, near Weta, in Jackson County, South Dakota. It measures 120 x 66 x 18 mm and weighs 196 grams.
Under short wave UV (254 nm), the middle calcite layer fluoresces orange/red, sandwiched between two layers of green fluorescent chalcedony. The green fluorescence of the chalcedony layers is activated by trace amounts of uranium (in the form of the uranyl ion).
Shown above under visible light, this flat specimen consists of pale yellow calcite sandwiched between two layers of light grey chalcedony.